Fuel cells have been proposed as a power source in a variety of vehicular applications, as well as other devices. One example of a fuel cell is the proton exchange membrane (PEM) fuel cell. PEM fuel cells include membrane-electrode-assemblies (MEAs) having a thin, proton-conductive, membrane-electrolyte with an anode electrode film formed on one face thereof and a cathode electrode film formed on the opposite face thereof. The MEA may also include a diffusion media for dispersing the reactant gases across the catalytic faces. The membrane-electrode assembly is sandwiched between a pair of electrically-conductive bipolar plate elements which serve as a current collector for the anode/cathode of the fuel cell and contain a plurality of lands and channels in the faces thereof for distributing the fuel cell's gaseous reactants (e.g., hydrogen and oxygen/air).
Each bipolar plate serves as an electrical conductor between adjacent fuel cells and is provided with a coolant flowing within a plurality of internal heat exchange passages to remove heat from the fuel cell. The common bipolar plate is an assembly constructed by joining two separate metal sheets or plates each having external facing reactant gas channels and internal facing coolant channels. In order to conduct electrical current between the anode portion of one cell and the cathode portion of the next adjacent cell in the fuel cell, the paired plates forming each bipolar plate assembly are mechanically and electrically joined.
Several methods to join bipolar plates are well known. In an exemplary application, the U.S. Pat. No. 5,776,624 issued to Neutzler provides a plurality of lands, which are mechanical connection points between plate pairs forming a bipolar plate. The plurality of lands in the Neutzler device are joined by a brazing process wherein the material used for brazing is carefully controlled to limit the insoluble metal which can leach from the brazed joints. The Neutzler brazing technique is effective at electrically joining adjacent plates of a bipolar plate assembly, but, difficult and costly to ensure a sufficient bond between the plates. Thus, an improvement providing a less expensive and less material critical joining method is desirable.
To limit the leaching problem identified above, brazed joints between the plates of a bipolar plate assembly are replaced by welded joints. In order to maintain the necessary metal-to-metal contact for welding, and, to ensure that the proper welding gap is provided, external pressure plates are commonly used to clamp the plates together and physically hold them during the period of time when welding takes place. Several drawbacks to the pressure plate welding method exist. First, a plurality of apertures or access holes must be included within the pressure plate(s) to provide access for the welding torch and welding beam (e.g., laser welding) to contact the desired surfaces of the plates. These apertures increase the cost and complexity for welding bipolar plates, particularly for complex bipolar plate channel and land geometries. A pressure plate prepared for a complex geometry of channels and lands generally can only be used for that design alone, which requires multiple pressure plate designs to accommodate various bipolar plate designs. This decreases the opportunity to use a particular set of pressure plates for welding more than one bipolar plate design because the arrangement of apertures in a pressure plate is highly dependent on the configuration of channels and lands on the individual plates forming each bipolar plate.
Another drawback of the pressure plate welding method results from the contact pressure adjacent to the individual welding sites which is lost by providing the welding apertures themselves. The pressure required to maintain clearance for welding is not significant; however, localized gaps between the paired plates forming the bipolar plates can occur where the apertures for welding do not provide sufficient force to maintain the paired plates in contact for welding.
A further drawback of the pressure plate welding method results because the plate thickness of the pressure plate increases the welding head separation from the welded surfaces. In a laser welding application, increasing this distance normally requires the addition of a special lens having a longer focal length and a smaller working angle which increases the cost of such a system. Also, a reduced percentage of acceptable weld joints can result.
It is therefore desirable to provide a method and system for clamping and welding pairs of plates to form bipolar plates which eliminates the need for pressure plates and therefore the expense and limitations of the pressure plate design. It is also desirable to provide a method and system for clamping and welding bipolar plates which reduces yet accommodates the occurrence of localized gaps between plate pairs to improve bipolar plate joining.